Digital cameras that have been popular in recent years obtain an image in the form of digital data. Unlike the photographic images, the digital image data can be subjected to various image processing that allows so-called retouching to modify the image and correct color in a later time. It is also a common technique during image capture to detect human faces in a picture frame and control to have a proper face size and appropriate brightness (see, for example, Japanese Patent Laid-open Publication No. 2004-320284 corresponding to U.S. Patent Application Publication No. 2004/0207743).
Meanwhile, when a subject image is formed on a photographic film or an image sensor through a lens, the image is sometimes blurred or distorted. A main cause of such blur and distortion is aberration of the lens used. In particular, the blur is caused by spherical aberration, astigmatism, coma and curvature of field.
The distortion of image, on the other hand, is caused by distortion aberration. Since the distortion aberration is a phenomenon in which the incident light is more refracted with distance from a lens center, the resultant distortion (hereinafter, optical distortion) of image changes according to positional relationship between a lens and an aperture stop. Therefore, in a zoom lens whose aperture stop changes the relative position to the lens system, it is difficult to reduce the optical distortion uniformly.
For film cameras, there would be no choice in reducing the optical distortion but to arrange the layout of a photographic film or the design of a taking lens. For digital cameras, however, the optical distortion can be reduced by applying an electrical image conversion processing to image data. For example, there are known a video camera to display different portions of an image with different magnifications (see, for example, Japanese Laid-open Publication No. 09-018763), and a digital camera to reduce the optical distortion of a captured image by using an image conversion processing (see, for example, Japanese Patent Laid-open Publication No. 11-252431).
Apart from the optical distortion, there is also known wide-angle distortion. The wide-angle distortion is a phenomenon that arises when a three-dimensional subject is projected through a lens onto a flat plane (a light receiving surface of an image sensor or a photographic film), and has an effect to elongate the image radially from the center of the picture frame. Therefore, the wide-angle distortion usually becomes apparent in wide-angle images, and is more prominent in the periphery of picture frame.
Since it results from a basic principle of the imaging apparatus, the wide-angle distortion cannot be eliminated completely. However, it is known that the wide-angle distortion becomes less noticeable when a lens with large distortion aberration is used (see, for example, Japanese Patent Laid-open Publication No. 11-065033).
Conventionally, a popular lens has a 45 degree angle of view, and the wide-angle distortion has not been a problem. In recent years, however, the wide-angle lenses with a 70 degree or more angle of view become popular, and loss of image quality due to the wide-angle distortion grows into a serious problem. For example, if a group image is captured with this type of wide-angle lens, the person at the periphery of the picture frame may be saddled with a distorted face due to the wide-angle distortion.
Nonetheless, if a lens with large distortion aberration is employed to minimize the wide-angle distortion, the optical distortion will be promoted excessively in a captured image. For example, capturing a building or such a rectilinear object with this type of lens will only produce unnatural images where the straight lines are curved. This is not a problem to special effect cameras, such as the lens-fitted film unit of the Publication No. 11-065033, but a serious problem to normal digital cameras.